The current development towards ever smaller and lighter optical systems demands novel approaches for the technical realization of optical devices for these intended applications. Especially heavy and slow optical devices based on glass and/or solid lens systems which are moved back and forward for focusing or zooming are sought to be replaced by so called liquid tuneable lens systems. The main part of such lens systems is a liquid-filled core element comprising a rigid container and a deformable membrane, which encloses a so-called “optical liquid”. The lens is typically formed by a lens shaper, which defines the optically clear aperture of the lens and the initial deformation of the lens. In case of manually tuneable lenses (ML), the shaper is pushed directly into the membrane by hand/manually, whereas in case of electrically tuneable lenses (EL), an electromagnetically actuated bobbin is pushed into the membrane, deforming the liquid filled volume and therefore changing the curvature of the lens as a function of the applied force to the actuated bobbin.
The optical liquid is enclosed by a soft membrane and a rigid container. The initial shape of the lens is defined by the lens shaper and the amount of optical liquid in the volume. When a bobbin is pushed into the deformable membrane, fluid is pumped from the surrounding of the lens into the centre, resulting in a deformation of the central part of the lens and therefore in a change of the focal length of the lens.
The optical liquid is a crucial component of the liquid tuneable lens system. In particular, it is desirable that the optical liquids have high refractive index (1.5), Abbe's number ≥40, transparency in the visible range (400-800 nm) of at least 90%, low volatility, wide operational range of temperature from desirably about −20° C. to +100° C. and chemical compatibility with other components of the lens system (e. g. membranes, container materials, glues). Further, the viscosity of the optical liquids should be preferably not higher than 5000 mPas.
However, materials having high Abbe's numbers ν (νd=(nd−1)/(nF−nc) with refractive indices nd at 587.56 nm, nF at 486.1 nm, nc at 656.3 nm or νD=(nD−1)/(nF−nc) with refractive indices nD at 589.3 nm, nF at 486.1 nm, nc at 656.3 nm) as well as high refractive indices n (n=sin αi/sin αr), i meaning angle of incident light and r angle of reflective light) are not easy to obtain because said features normally cannot be enhanced in parallel. Thus, the materials presently available and which are used as optical liquids represent a compromise between those features.
In the art, several attempts for providing materials fulfilling at least some of these requirements have been proposed. For example, commercially available fluorocarbon/perfluoropolyether fluids (DuPont, Nye, Solvay Specialty Polymers) show Abbe's numbers of ν>100 but have refractive indices of n≈1.3. Recently, a new class of polythio ether sulfones based compounds exhibiting high Abbe's numbers and high refractive indices have been reported by Y. Suzuki et al. Macromolecules, 2012, 45, 3402. The polythio ether sulfones based compounds are of the following formula:
However, the afore-mentioned materials are solids at room temperature including their monomeric building blocks and are therefore not suitable as optical liquids in liquid tuneable lens systems.
Another class of materials has been disclosed by T. Okubo et al. J. Appl. Polmer Sci., 1998, 68, 1791 and T. Okubo et al. J. Mater Sci., 1999, 34, 337. Further, materials based on the thio phenol ether motif (n>1.6) are proposed as ingredients. The thio phenol ether motif of the materials is as follows:

However, these compounds have Abbe's number of 23 and are only available as mixtures and are very costly. In addition the quality is variable and available amounts are low. Other materials containing an oligo-sulfide motif and additionally reactive acrylic residues are used to produce polymers with high nD and ν (see e.g. DE 4011868, EP 0 284 374 A2).
Thus, there is a need in the art for providing liquids which are suitable as optical liquids, especially in liquid tuneable lens systems. In particular, it is desirable to provide a liquid having high Abbe's number ν as well as high refractive index n. A high refractive index is needed for a high optical power whereas a high Abbe number is beneficial for low chromatic aberration/dispersion (dependency of the wave length on the refractive index).
Accordingly, it is an object of the present invention to provide a liquid which is suitable as optical liquid. Furthermore, it is an object of the present invention to provide a liquid which is suitable as optical liquid in liquid tuneable lens systems. It is an even further object of the present invention to provide a liquid having an improved balance of (high) Abbe's number ν and (high) refractive index n (refractive index≈1.27 and higher, Abbe's number≈35 and higher). A still further object of the present invention is to provide a liquid having high transmittance in the visible range. An even further object of the present invention is to provide a liquid having good compatibility with the membrane materials. Further objects can be gathered from the following description of the invention.